In recent years, rechargeable electric storage devices such as battery cells (lithium-ion cells, nickel-hydrogen cells, or the like) and capacitors (electric double layer capacitors or the like) have been adopted as motive power sources for vehicles (motor vehicles, motorcycles or the like) or various devices (portable terminals, notebook computers or the like). As motive power sources for electric vehicles or the like in particular, various lithium secondary cells having high energy densities have been provided. Such a lithium secondary cell includes a cell case made of a metal, a power generating element housed in the cell case, electrode terminals projecting from the inside to the outside of the cell case, and current collectors that connect the power generating element and the electrode terminals. Patent Document 1 describes the invention of a prismatic secondary cell having a power generating element and current collectors connected to each other without increasing the internal resistance of the cell.
In this prismatic secondary cell, as shown in FIG. 11, a power generating element (“wound electrode assembly” in Patent Document 1) 120 is housed in a cell case (“cell can” in Patent Document 1) 110, with gaps provided between the power generating element 120 and inner surfaces of the cell case 110; positive and negative electrode terminals 131 and 132 project from the inside to the outside of the cell case 110; and the power generating element 120 is connected to the electrode terminals 131 and 132 by current collectors (“current collecting members” in Patent Document 1) 140.
The power generating element 120 has a separator in band form interposed between positive and negative electrode plates each in band form (not individually illustrated), in which the separator and the positive and negative electrode plates are spirally wound and compressed into a flat shape. The power generating element 120 has central holes 121 such as shown in FIG. 12 (only one of them is illustrated in FIG. 12) at a winding center between its opposite end portions. The power generating element 120 also has core bundle portions 122 at its opposite sides. The core bundle portions 122 are projecting end portions of positive and negative cores through the respective sides. The power generating element 120 is housed in the cell case 110, with the central holes 121 facing end surfaces of the cell case 110, and with the core bundle portions 122 positioned at the opposite ends of the cell case 110.
Each current collector 140 has a terminal connection portion 141 welded to the electrode terminal 131 or 132, a core connection portion 142 inserted in the central hole 121 of the power generating element 120, and a link portion 143 that links the terminal connection portion 141 and the core connection portion 142. The current collector 140 is formed by pressing one rectangular metal plate.
The link portion 143 has a width slightly smaller than the width in the compression direction of the power generating element 120. The link portion 143 is bent orthogonally to the core connection portion 142 (to be L-shaped in the front view). The core connection portion 142 is folded in half along its longitudinal center line. The ridge of the fold is projecting in the direction of bending of the terminal connection portion 141.
The core connection portion 142 is inserted in the central hole 121 of the power generating element 120, and the core bundle portion 122 of the power generating element 120 is pressed, thereby bringing the core connection portion 142 and the power generating element 120 into pressure contact with each other. In this state, ultrasonic welding is performed from one outer peripheral side of the core bundle portion 122 to connect the core connection portion 142 and the inner peripheral surface of the core bundle portion 122 without increasing the internal resistance.
The cell case 110 is a combination of a case body 111 in the form of a bottomed rectangular cylinder having an opening, and a cover plate 112 with which the opening of the case body 111 is closed. End edges of the opening of the case body 111 and outer peripheral edges of the cover plate 112 are welded to each other. Holes (not numbered) through which the electrode terminals 131 and 132 are passed are provided in the cover plate 112. The electrode terminals 131 and 132 project through the holes of the cover plate 112 and are fixed on the cover plate 112 by rivets. Electric power generated in the power generating element 120 is taken out through the electrode terminals 131 and 132 projecting from the cover plate 112.
In the conventional cell, gaps are provided between the power generating element 120 and the inner surfaces of the case body 111 of the cell case 110. Therefore the power generating element 120 is in a state of being suspended from the cover plate 112 by the current collectors 140. Also, the terminal connection portions 141 of the current collectors 140 are fixed to the cover plate 112 of the cell case 110.
Therefore, when vibration occurs in the cell mounted, for example, on a motor vehicle during traveling, the power generating element 120 vibrates with an amplitude and a frequency different from those of the vibration of the case body 111. Stress is then caused in the portions where the current collectors 140 is connected to the power generating element 120 and the portions where the current collectors 140 is bent (link portions 143), resulting in breakage of these portions and inoperability of the cell in some cases.
A repetitive test based on the vibration test profile in accordance with the UN Transport Test was performed on a current collector similar in form to the current collector 140 described in Patent Document 1 by successively increasing the parameter that defines the maximum acceleration to 8, 10, 12, 15, 18, 20, and 22 G. Breakage was caused in some samples at 16 G and 8 G.
In the conventional cell, not only welding of the current collectors 140 to the power generating element 120 and to the electrode terminals 131 and 132 but also welding between the case body 111 and the cover plate 112 of the cell case 110 is performed. Therefore, each current collector 140 cannot be replaced by being taken out after the current collector 140 is broken.
There is the same kind of problem with current collectors in capacitors (electric double layer capacitors or the like).